Antibodies to nucleic acids in patients with Systemic Lupus Erythematosus and in New Zealand Black mice who spontaneously develop a lupus-like syndrome are being studied by a filter radioimmunoassay method. Antibodies to DNA, RNA and to a DNA:RNA hybrid are assayed. The major investigation involves analysis of these various activities after fractionation of serum by sucrose density gradient ultracentrifugation. antibodies to RNA tend to be both 19 and 7S, whereas antibodies to DNA are predominantly 7S. Antibodies to the DNA:RNA hybrid are found in both 19S and 7S fractions.